Non-degenerate polarization-entangled photon pair generation device and non-degenerate polarization-entangled photon pair generation method

ABSTRACT

A non-degenerate polarization-entangled photon pair generation device ( 1 ) that efficiently and easily generates non-degenerate polarization-entangled photon pairs includes: a quantum-entangled photon pair generator ( 2 ) including a single crystal in which periodically poled structures ( 3   a,    3   b ) having different periods are formed; and a light radiating unit ( 4 ) for entering light into the quantum-entangled photon pair generator ( 2 ) such that the light passes through the periodically poled structure ( 3   a ) and then through the periodically poled structure ( 3   b ). A period of the periodically poled structure ( 3   a ) is different from a period of the periodically poled structure ( 3   b ) such that a parabola indicative of a relation between an emission angle and a wavelength of a polarized photon emitted based on light incident on the periodically poled structure ( 3   a ) comes into contact with a parabola indicative of a relation between an emission angle and a wavelength of polarized photon emitted based on light incident on the periodically poled structure ( 3   b ), within an allowable range under a phase matching condition.

TECHNICAL FIELD

The present invention relates to a non-degenerate polarization-entangledphoton pair generation device and a non-degeneratepolarization-entangled photon pair generation method each for generatingphoton pairs in a non-degenerate polarization-entangled state.

BACKGROUND ART

In recent years, information communication techniques more widely havebeen implemented in the form of, for example, electronic commerce andelectronic mails. To cope with this, cryptographic techniques ininformation transmission have been also researched and developed. As oneof the cryptographic techniques, quantum cryptography is highly expectedrecently.

In the quantum cryptography, security is ensured by utilizing a physicalphenomenon according to Heisenberg's uncertainty principle in quantummechanics. According to the uncertainty principle, a quantum state ischanged by observation, and therefore, eavesdropping (observation) of acommunication cannot be performed without being surely detected. Thisallows taking measures against the eavesdropping, such as blocking thecommunication. This makes eavesdropping physically impossible in thequantum cryptography. Further, according to the uncertainty principle,replication of particles is also impossible in the quantum cryptography.

Quantum teleportation is an important feature in the quantumcryptography. The quantum teleportation is a technique for transmittingonly quantum information of the particles to another place. The quantumteleportation is realized by exchanging information between photons byutilizing a quantum-entangled state. A photon pair in thequantum-entangled state has such a property that a quantum state of oneof the photons is determined when a quantum state of the other one ofthe photons is determined. This property is not dependent on a distancebetween the two photons.

In the quantum teleportation technique, such photon pairs in thequantum-entangled state are essential.

The following describes a two-photon polarization-entangled state. It isknown that a quantum-entangled state of 2 quantum bits (two photons)using polarized light takes the following 4 states.

Math.  1 $\begin{matrix}\left. \left. \left. {\left. {\left. \left| \Psi^{\pm} \right. \right\rangle_{12} \equiv {\frac{1}{\sqrt{2}}\left( \left| H \right. \right\rangle_{1}}} \middle| V \right\rangle_{2} \pm} \middle| V \right\rangle_{1} \middle| H \right\rangle_{2} \right) & (1) \\\left. \left. \left. {\left. {\left. \left| \Phi^{\pm} \right. \right\rangle_{12} \equiv {\frac{1}{\sqrt{2}}\left( \left| H \right. \right\rangle_{1}}} \middle| H \right\rangle_{2} \pm} \middle| V \right\rangle_{1} \middle| V \right\rangle_{2} \right) & (2)\end{matrix}$

A light path of the photon and an angular frequency of the photon aresome of physical quantities to determine a mode i of a photon.

The following describes a method (parametric down-conversion) forproducing two photons. As a physical process to produce a two-photonstate, a parametric down-conversion process is often used. In theparametric down-conversion process, a single pump photon (angularfrequency ω_(p), wave vector k_(p)) incident on a crystal is convertedinto a photon pair with a certain probability. One of the photon pair isa signal photon (angular frequency ω_(s), wave vector k_(s)) and, theother one of the photon pair is an idler photon (angular frequencyω_(i), wave vector k_(i)). At this time, in order that the parametricdown-conversion process may be caused, the following phase matchingcondition should be satisfied.

Math. 2ω_(p)=ω_(s)+ω_(i)  (3)k _(p) =k _(s) +k _(i)  (4)

There are 3 types of the phase matching conditions depending onpolarization of the photons.

-   1. Type-O Phase Matching Condition-   This is a case where the pump photon, the signal photon, and the    idler photon have the same polarization.-   2. Type-I Phase Matching Condition-   This is a case where the signal photon and the idler photon have the    same polarization, and the polarization of the pump photon is    perpendicular to the polarization of the signal photon and the idler    photon.-   3. Type-II Phase Matching Condition-   This is a case where the polarization of the signal photon is    perpendicular to the polarization of the idler photon, and the pump    photon has the same polarization as either of the polarization of    the signal photon and the polarization of the idler photon.

Next will be explained a quasi phase matching method. The quasi phasematching method is well known as a technique for satisfying the phasematching condition at a certain wavelength. In the quasi phase matchingmethod, a second order nonlinear optical susceptibility is periodicallymodulated so as to satisfy the phase matching condition. In this case,the above expression (4) of the phase matching condition is changed tothe following expression (5):

Math.  3 $\begin{matrix}{k_{p} = {k_{s} + k_{i} + \frac{2\pi}{\Lambda}}} & (5)\end{matrix}$where Λ is a modulation period of the second order nonlinear opticalsusceptibility. A “periodic polarization reversal method” in whichspontaneous polarization of a crystal is periodically reversed is putinto practice as a technique for periodically modulating the secondorder nonlinear optical susceptibility.

The following describes a conventional method for producing apolarization-entangled state. There have been reported severaltechniques as the method for producing a polarization-entangled state inwhich two photons have the same angular frequency (for example, see NonPatent Literature 1). In this method, since the two photons have thesame angular frequency and therefore it is difficult to distinguish themfrom each other, a mode is determined according to a light path of thephoton. That is, the two photons should be emitted in different lightpaths.

Further, there has been also suggested a method for producing apolarization-entangled state in which two photons have different angularfrequencies. In this method, since the photons are distinguished fromeach other according to the angular frequencies, the two photons may beemitted in the same light path.

As the method for producing a polarization-entangled photon pairconstituted by two photons having different angular frequencies, therehave been reported the following methods.

1. A Method Utilizing a Parametric Down-Conversion of Type-O or Type-I(Non Patent Literature 2)

-   This method utilizes nonlinear optical crystals that satisfy the    phase matching conditions type-O or type-I so as to generate two    photons having the same polarization state. The nonlinear optical    crystals are aligned in series by being rotated in opposite    directions by 90 degrees. In this case, the two crystals are    irradiated by light from the same pump light source, so as to    generate two photons (ω₁, ω₂) having different angular frequencies    in a coaxial direction of pump light.

2. A Method for Producing Two Types of Periodically Poled Structures ina Single Crystal (Patent Literature 2)

-   This method employs different phase matching conditions type-O and    type-I.

3. A Method Utilizing a Four-Wave Mixing Process that is a ThirdNonlinear Optical Phenomenon Caused in Optical Fibers (Non PatentLiterature 3)

-   In this method, optical fibers are set in an interferometer and a    polarization-entangled state is generated.

Further, a method in which non-degenerate polarization-entangled photonpairs are produced by utilizing a two-photon resonance excitationprocess in a semiconductor has been also known (Patent Literature 1).

Citation List

Patent Literature 1

Japanese Patent Application Publication, Tokukai, No. 2005-309012 A(Publication Date: Nov. 4, 2005)

Patent Literature 2

Japanese Patent Application Publication, Tokukai, No. 2007-114464 A(Publication Date: May 10, 2007)

Non Patent Literature 1

“New high-intensity source of polarization-entangled photon pairs.” P.G. Kwiat et al., Phys. Rev. Lett. 75, 4337 (1995).

Non Patent Literature 2

“Bright, single-spatial-mode source of frequency non-degenerate,polarization-entangled photon pairs using periodically poled KTP.” M.Pelton et al., Opt. Express 12, 3573 (2004).

Non Patent Literature 3

“Generation of polarization-entangled photon pairs and violation ofBell's inequality using spontaneous four-wave mixing in a fiber loop,”H. Takesue and Kyo Inoue, Phys. Rev. A 70, 031802 (2004).

SUMMARY OF INVENTION

However, the method disclosed in Non Patent Literature 2 is complicatedbecause it is necessary to prepare two crystals having the same propertyand to align them precisely.

The method disclosed in Patent Literature 2 employs the different phasematching conditions type-0 and type I, which causes a problem that it isdifficult to take a balance in generation efficiency between two photonsgenerated from respective periodically poled structures of the type-0and type-I phase matching conditions. The imbalance in the generationefficiency between two photons generated from the respectiveperiodically poled structures decreases a degree of quantumentanglement.

In the method disclosed in Non Patent Literature 3, it is necessary toset optical fibers in an interferometer for the generation of thepolarization-entangled state. This makes a configuration of a devicemore complicated.

Further, the method disclosed in Patent Literature 1 uses a resonantlevel in a semiconductor, and therefore is limited as to an angularfrequency of pump light to be incident. As a result, a frequency band ofgenerated non-degenerate polarization-entangled photon pairs is alsolimited to a narrow range.

The present invention is accomplished in view of the above problems. Anobject of the present invention is to realize a non-degeneratepolarization-entangled photon pair generation device which can easilygenerate non-degenerate polarization-entangled photon pairs and whichcan improve efficiency of generating the non-degeneratepolarization-entangled photon pairs.

In order to achieve the above object, a non-degeneratepolarization-entangled photon pair generation device of the presentinvention includes: a quantum-entangled photon pair generator includinga single crystal in which a first periodically poled structure having afirst period and a second periodically poled structure having a secondperiod that is different from the first period are formed; and lightradiating means for entering light into the single crystal such that thelight passes through the first periodically poled structure and thenthrough the second periodically poled structure.

With the configuration, it is possible to form, in a single crystal, afirst periodically poled structure and a second periodically poledstructure, each for generating non-degenerate polarization-entangledphoton pairs. This makes it possible to easily generate thenon-degenerate polarization-entangled photon pairs and to increasegeneration efficiency of the non-degenerate polarization-entangledphoton pairs.

In addition, since the present invention does not utilize a resonantlevel in a substance, a limitation on an angular frequency of pump lightto be incident on the single crystal is moderate. As a result, it ispossible to generate non-degenerate polarization-entangled photon pairsin a large bandwidth.

In the non-degenerate polarization-entangled photon pair generationdevice of the present invention, it is preferable that the first periodof the first periodically poled structure be different from the secondperiod of the second periodically poled structure such that a parabolaindicative of a relation between an emission angle and a wavelength of apolarized photon emitted based on light incident on the firstperiodically poled structure comes into contact with a parabolaindicative of a relation between an emission angle and a wavelength ofpolarized photon emitted based on light incident on the secondperiodically poled structure, within an allowable range under a phasematching condition.

With the above configuration, it is possible to adjust poling periods toa type-II phase matching condition that generates such a pair of photonsthat one of the photons has an angular frequency ω1 and the other one ofthe photons has an angular frequency ω2 in a coaxial direction of atraveling direction of the pump light and polarization of the one of thephotons is perpendicular to polarization of the other one of thephotons.

In the non-degenerate polarization-entangled photon pair generationdevice of the present invention, it is possible to use a lithium niobatecrystal as the single crystal.

With the above configuration, it is possible to easily form the firstperiodically poled structure and the second periodically poled structureeach for generating non-degenerate polarization-entangled photon pairs.

It is preferable that the non-degenerate polarization-entangled photonpair generation device of the present invention further include a thirdperiodically poled structure that produces pump light, and the lightradiating means enter light into the third periodically poled structuresuch that the light passes through the third periodically poledstructure, the first periodically poled structure, and the secondperiodically poled structure, in this order.

In the above configuration, the light emitted from the light radiatingmeans enters the third periodically poled structure so that the light isconverted into pump light. The pump light then enters the first andsecond periodically poled structures so that non-degeneratepolarization-entangled photon pairs are generated.

In the non-degenerate polarization-entangled photon pair generationdevice of the present invention, it is preferable that the thirdperiodically poled structure be formed in the single crystal.

In the above configuration, it is not necessary to prepare anothercrystal to generate pump light. Accordingly, a simple configuration forgenerating non-degenerate polarization-entangled photon pairs can beobtained.

In order to achieve the above object, a method of the present inventionfor generating non-degenerate polarization-entangled photon pairsincludes the step of entering light into a single crystal in which afirst periodically poled structure having a first period and a secondperiodically poled structure having a second period different from thefirst period are formed, in such a manner that the light passes throughthe first periodically poled structure and then through the secondperiodically poled structure.

With the above configuration, it is possible to form a firstperiodically poled structure and a second periodically poled structure,each for generating non-degenerate polarization-entangled photon pairs,in a single crystal. This makes it possible to easily generate thenon-degenerate polarization-entangled photon pairs and to increasegeneration efficiency of the non-degenerate polarization-entangledphoton pairs.

As described above, the non-degenerate polarization-entangled photonpair generation device of the present invention includes aquantum-entangled photon pair generator including a single crystal inwhich first and second periodically poled structures having differentperiods are formed. This attains advantageous effects thatnon-degenerate polarization-entangled photon pairs can be easilygenerated and generation efficiency of the non-degeneratepolarization-entangled photon pairs can be increased.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically illustrating a configuration of anon-degenerate polarization-entangled photon pair generation deviceaccording to one embodiment.

FIG. 2 is a view schematically illustrating a configuration of anothernon-degenerate polarization-entangled photon pair generation deviceaccording to one embodiment.

FIG. 3 is a view schematically illustrating an example of thenon-degenerate polarization-entangled photon pair generation device.

FIG. 4 is a graph showing a relation between an emission angle and awavelength of a polarized photon emitted based on light incident on afirst periodically poled structure provided in the non-degeneratepolarization-entangled photon pair generation device.

FIG. 5 is a graph showing a relation between an emission angle and awavelength of a polarized photon emitted based on light incident on asecond periodically poled structure provided in the non-degeneratepolarization-entangled photon pair generation device.

FIG. 6 shows the graph in FIG. 4 and the graph in FIG. 5 together insuch a manner that the graphs in FIG. 4 and FIG. 5 are overlapped witheach other.

FIG. 7 is a view schematically illustrating an example of the anothernon-degenerate polarization-entangled photon pair generation device.

FIG. 8 is a view schematically illustrating an example of thenon-degenerate polarization-entangled photon pair generation device.

FIG. 9 is a graph showing a temperature dependency of a parametricfluorescence spectrum in the example.

REFERENCE SIGNS LIST

-   1 Non-Degenerate Polarization-Entangled Photon Pair Generation    Device-   2 Quantum-Entangled Photon Pair Generator-   3 a Periodic Poling Structure (First Periodically Poled Structure)-   3 b Periodic Poling Structure (Second Periodically Poled Structure)-   3 c Periodic Poling Structure (Third Periodically Poled Structure)-   4 Light Radiating Unit (Light Radiating Means)

DESCRIPTION OF EMBODIMENTS

One embodiment of the present invention is explained below withreference to FIG. 1 through FIG. 7.

In the present embodiment, periodically poled structures different fromeach other are formed in a single crystal under a type-II phase matchingcondition that generates two photons whose polarization is orthogonal toeach other. In this case, a poling period of one of the periodicallypoled structures is adjusted such that a photon having an angularfrequency ω₁ has first polarization and a photon having an angularfrequency ω₂ has second polarization. Further, a poling period of theother one of the periodically poled structures is adjusted such that aphoton having an angular frequency ω₁ has second polarization and aphoton having an angular frequency ω₂ has first polarization. With thesestructures, a polarization-entangled state at non-degenerate wavelengthsis generated from a single crystal, by utilizing a frequency mode. Thepresent invention uses the type-II phase matching condition for eitherof the periodically poled structures, thereby making it easy to take abalance of two-photon generation efficiency between the periodicallypoled structures. Further, this configuration does not require anadditional interferometer, thereby successfully providing a very simpledevice configuration.

FIG. 1 is a view schematically illustrating a configuration of anon-degenerate polarization-entangled photon pair generation device 1according to the present invention. The non-degeneratepolarization-entangled photon pair generation device 1 includes aquantum-entangled photon pair generator 2. The quantum-entangled photonpair generator 2 is constituted by a single crystal in whichperiodically poled structures 3 a and 3 b having different periods areformed. The single crystal is made of a lithium niobate crystal. Thenon-degenerate polarization-entangled photon pair generation device 1also includes an incidence unit 4 that causes light to enter the photonpair generator 2 (single crystal) in such a manner that the light passesthrough the periodically poled structure 3 a and then through theperiodically poled structure 3 b.

In the present embodiment, the periodically poled structures 3 a and 3 bhaving different periods are formed in the single crystal (the photonpair generator 2) (FIG. 1). Poling periods are adjusted to satisfy thetype-II phase matching condition. Under the type-II phase matchingcondition, such a pair of photons is generated, with respect to incomingpump light of an angular frequency ω₃ and a wave number vector k₃, in acoaxial direction of a traveling direction of the incoming pump lightthat one of the photons has an angular frequency ω₁ (wave number vectork₁) and the other one of the photons has an angular frequency ω₂ (wavenumber vector k₂), and polarization of the one of the photons isperpendicular to that of the other one of the photons. In quasi phasematching employing such periodically poled structures, the angularfrequencies and the wave number vectors in the light are respectivelyrepresented by the following relational expressions.

Math.  4 $\begin{matrix}{\omega_{3} = {\omega_{1} + \omega_{2}}} & (6) \\{k_{3} = {k_{1} + k_{2} + \frac{2\pi}{\Lambda_{i}}}} & (7)\end{matrix}$

In the expression (7), Λ_(i) indicates a poling period. In this case, apoling period Λ₁ is adjusted so that the first periodically poledstructure 3 a generates a photon pair such that a photon having anangular frequency ω₁ has first polarization and a photon having anangular frequency ω₂ has second polarization. In the first periodicallypoled structure 3 a, a spontaneous polarization region that the crystaloriginally has and a spontaneous-polarization-reversed region where thespontaneous polarization is reversed are provided in an alternate manneralong a traveling direction of the photons. The poling period Λ₁indicates a period in which one cycle is a set of the spontaneouspolarization region and the spontaneous-polarization-reversed region.Further, a poling period Λ₂ is adjusted so that the second periodicallypoled structure 3 b generates a photon pair such that a photon having anangular frequency ω₁ has second polarization and a photon having anangular frequency ω₂ has first polarization. In the second periodicallypoled structure 3 b, a spontaneous polarization region that the crystaloriginally has and a spontaneous-polarization-reversed region where thespontaneous polarization is reversed are provided in an alternate manneralong a traveling direction of the photons. The poling period Λ₂indicates a period in which one cycle is a set of the spontaneouspolarization region and the spontaneous-polarization-reversed region.

In a case where single pump light is supplied to a quasi phase matchingelement configured as such, a generated two-photon state forms a staterepresented by the following expression:

Math.  7 $\begin{matrix}\left. \left. \left. {\left. {\left. \left| \psi \right. \right\rangle = \left. {\frac{1}{\sqrt{2}}\left( \left| H \right. \right\rangle_{\omega 1}} \middle| V \right.} \right\rangle_{\omega 2} + {\mathbb{e}}^{i\;\phi}} \middle| V \right\rangle_{\omega 1} \middle| H \right\rangle_{\omega 2} \right) & (8)\end{matrix}$The two-photon state represented by the above expression is a linearsuperposition of states of (i) a two-photon state produced by theperiodically poled structure 3 a, represented by the followingexpression:Math. 5|H

_(ω1) |V

_(ω2)and (ii) a two-photon state produced by the periodically poled structure3 b represented by the following expression:Math. 6|V

_(ω1) |H

_(ω2)In the expression (8), φ is a phase different between the two-photonstates respectively generated from the two periodically poled structures3 a and 3 b. The state represented by the expression (8) is apolarization-entangled state utilizing a frequency mode.

FIG. 2 is a view schematically illustrating another non-degeneratepolarization-entangled photon pair generation device 1 a according tothe present embodiment. The non-degenerate polarization-entangled photonpair generation device 1 a carries out second harmonic generation withthe use of quasi phase matching by a periodically poled structure sothat pump light having an angular frequency ω₃ is generated. That is, aperiodically poled structure 3 c that satisfies the followingexpressions is formed as a zeroth period (FIG. 2).

Math.  8 $\begin{matrix}{\omega_{3} = {2\;\omega_{0}}} & (9) \\{k_{3} = {{2k_{0}} + \frac{2\pi}{\Lambda_{0}}}} & (10)\end{matrix}$where ω₀ is an angular frequency of a fundamental wave in the secondharmonic generation and k₀ is a wave number vector of the fundamentalwave. This structure allows a single crystal to generatepolarization-entangled state of two photons respectively having anangular frequency ω₁ and an angular frequency ω₂, from incident lighthaving the angular frequency ω₀.

In the periodically poled structure 3 c, a spontaneous polarizationregion and a spontaneous-polarization-reversed region where thespontaneous polarization is reversed are provided in an alternate manneralong a traveling direction of the photons. A poling period Λ₀ indicatesa period in which one cycle is a set of the spontaneous polarizationregion and the spontaneous-polarization-reversed region.

The conventional technique requires two crystals or alternatively has touse an interferometer. In contrast, the method according to the presentembodiment does not require the interferometer. As a result, the methodaccording to the present embodiment has an advantage that non-degeneratepolarization-entangled photon pairs can be easily generated just by asingle crystal. Further, in order to carry out the second harmonicgeneration to obtain pump light for some frequency bands of two photonsto be generated, the conventional technique requires another crystal.However, in the present embodiment, the second harmonic generation toobtain the pump light and the parametric down-conversion to generate thetwo-photon state can be carried out by use of a single crystal.

FIG. 3 is a view schematically illustrating an example of thenon-degenerate polarization-entangled photon pair generation device 1.In the example, a lithium niobate crystal (LiNbO₃) is used as a crystalin which the periodically poled structures 3 a and 3 b are formed.Initially, a structure (period Λ₁=9.1 μm) is formed as the firstperiodically poled structure 3 a for generating two non-degeneratephotons having different wavelengths. Pump light having a wavelength of775 nm and being polarized in a y-axial direction is incident on thefirst periodically poled structure along an x axis of the crystal (FIG.3).

At this time, the first periodically poled structure emits a photonpolarized in the y-axial direction and a photon polarized in a z-axialdirection. FIG. 4 is a graph showing a relation between an emissionangle and a wavelength in each of the photons. The graph is obtained bysimulation.

The graph demonstrates that the photon polarized in the y-axialdirection and having a wavelength of 1580 nm, and the photon polarizedin the z-axial direction and having a wavelength of 1520 nm aregenerated in a coaxial direction (0 rad.) of the pump light.

Then, a structure (period Λ₁=9.3 μm) is formed as a second period. Thesame pump light as the one used in the first period structure isincident on the structure as the second period. At this time, thestructure generates a photon polarized in the y-axial direction and aphoton polarized in the z-axial direction. FIG. 5 is a graph showing arelation between an emission angle and a wavelength in each of thephotons. The graph is obtained by simulation. The graph demonstratesthat the photon polarized in the z-axial direction and having awavelength of 1580 nm and the photon polarized in the y-axial directionand having a wavelength of 1520 nm are generated in a coaxial direction(0 rad.) of the pump light.

FIG. 6 shows the graph in FIG. 4 and the graph in FIG. 5 together insuch a manner that the graphs in FIG. 4 and FIG. 5 are overlapped witheach other. The graph of FIG. 6 demonstrates that, within an allowablerange under the phase matching condition, (i) a parabola indicating thephoton polarized in the y-axial direction in FIG. 4 has contact with aparabola indicating the photon polarized in the z-axial direction inFIG. 5 at the wavelength of 1580 nm and (ii) a parabola indicating thephoton polarized in the z-axial direction in FIG. 4 has contact with aparabola indicating the photon polarized in the y-axial direction inFIG. 5 at the wavelength of 1520 nm. That is, it is demonstrated thatpolarization of photons indicated by parabolas having contact with eachother is perpendicular to each other. That is, in a case where the twoperiodically poled structures 3 a and 3 b are formed in a single crystaland the pump light polarized in the y-axial direction and having thewavelength of 775 nm is incident on the single crystal, the firstperiodically poled structure 3 a emits two photons of a photon having awavelength of 1580 nm and being polarized in the y-axial direction and aphoton having a wavelength of 1520 nm and being polarized in the z-axialdirection, which two photons are represented as follows:

Math. 9|t

₁₅₈₀ |z

₁₅₂₀On the other hand, the second periodically poled structure 3 b emits twophotons of a photon having a wavelength of 1580 nm and being polarizedin the z-axial direction and a photon having a wavelength of 1520 nm andbeing polarized in the y-axial direction, which two photons arerepresented as follows:Math. 10|z

₁₅₈₀ |y

₁₅₂₀Since the same pump light is incident on either of the structures, atwo-photon state to be emitted is a state where the above states arelinearly combined as represented as follows:

${{Math}.\mspace{14mu} 11}\begin{matrix}\left. \left. \left. {\left. {\left. \left| \psi \right. \right\rangle = \left. {\frac{1}{\sqrt{2}}\left( \left| y \right. \right\rangle_{1580}} \middle| z \right.} \right\rangle_{1520} + {\mathbb{e}}^{i\;\varphi}} \middle| z \right\rangle_{1580} \middle| y \right\rangle_{1520} \right) & (11)\end{matrix}$As such, it is demonstrated that a non-degenerate polarization-entangledstate using the same frequency mode as in the expression (1) isgenerated where the polarization in the y-axial direction is regarded asfirst polarization and the polarization in the z-axial direction isregarded as second polarization.

FIG. 7 is a view schematically illustrating an example of thenon-degenerate polarization-entangled photon pair generation device 1 a.In the non-degenerate polarization-entangled photon pair generationdevice 1 a, further another poling structure (Λ₀=16.1 μm) is formed as azeroth periodically poled structure 3 c in front of the periodicallypoled structures 3 a and 3 b (along a traveling direction of light).This makes it possible to obtain pump light having a wavelength of 775nm and being polarized in a y-axis direction, with respect to incidentlight having a wavelength of 1550 nm and being polarized in the y-axialdirection. As such, by adding the structure (the periodically poledstructure 3 c), it is possible to obtain a polarization-entangled photonpair of two photons from incident light having the wavelength of 1550 nmthat is a center of wavelengths of the two photons.

A polarization-entangled state obtained by the configuration of FIG. 3is not limited only to the wavelengths of 1580 nm and 1520 nm. Byadjusting the poling period, it is possible to obtain variouspolarization-entangled states of a variety of two wavelengths. Forexample, in a case where periodically poled structures of Λ₁=3.9 μm andΛ₂=4.9 μm are formed and pump light polarized in a y-axis direction andhaving a wavelength of 532 nm is incident on the periodically poledstructures along an x axis of a crystal, it is possible to obtain thefollowing polarization-entangled state of wavelengths of 810 nm and 1550nm.

Math.  12 $\begin{matrix}\left. \left. \left. {\left. {\left. \left| \psi \right. \right\rangle = \left. {\frac{1}{\sqrt{2}}\left( \left| y \right. \right\rangle_{810}} \middle| z \right.} \right\rangle_{1550} + {\mathbb{e}}^{i\;\varphi}} \middle| z \right\rangle_{810} \middle| y \right\rangle_{1550} \right) & (12)\end{matrix}$

Further, the polarization-entangled state obtained by the configurationof FIG. 3 is not limited to a case where the lithium niobate crystal isused. For example, it is also possible to obtain the non-degeneratepolarization-entangled state by use of a KTP crystal (KTiOPO₄) in whichsuch periodically poled structures can be produced. More specifically, anon-degenerate polarization-entangled state having wavelengths of 1580nm and 1520 nm, which is the same as the one represented by theexpression (9), can be obtained by forming periodically poled structureshaving a period Λ₁ of 49.8 μm and a period Λ₂ of 44.8 μm in the KTPcrystal (KTiOPO₄).

Further, as typical crystals, except for the lithium niobate (LiNbO₃)crystal, in which polarization can be reversed periodically, there havebeen known a lithium tantalate (LiTaO₃) crystal and a potassium titanylphosphate (KTiOPO₄). Potassium niobate (KNbO₃) is also a crystal inwhich polarization can be reversed.

FIG. 8 is a view schematically illustrating another example of thenon-degenerate polarization-entangled photon pair generation device 1,and briefly illustrates a PPLN crystal having two different periodicallypoled structures. Several techniques for generating non-degeneratepolarization-entangled photon pairs by use of a parametricdown-conversion (PDC) process have been reported. In these techniques,since a pair of photons having different wavelengths can be obtained, itis possible to easily and efficiently split the pair of photons by useof a mirror according to the wavelengths. In addition, by arranging suchthat one of the pair of photons has a wavelength of a communicationband, it is possible to transmit information farther. Moreover, theother one of the pair of photons can be at a near side so that itswavelength can be manipulated. In the present example, a type-II quasiphase matching element having different periods was structured in asingle LiNbO₃ (LN) crystal, and a spectrum of parametric fluorescencehaving two peak wavelengths in the communication band was observed.

A non-degenerate polarization-entangled photon pair generation device 1c includes a quantum-entangled photon pair generator 2 c. Thequantum-entangled photon pair generator 2 c is constituted by a singlecrystal in which a periodically poled structure 3 d having a period Λ₁of 9.25 (interaction length: 20 mm) and a periodically poled structure 3e having a period Λ₂ of 9.50 μm (interaction length: 20 mm). The singlecrystal is made of a lithium niobate crystal. The non-degeneratepolarization-entangled photon pair generation device 1 c also includesan incidence unit 4 that causes light to enter the photon pair generator2 c (single crystal) in such a manner that the light passes through theperiodically poled structure 3 d and then through the periodically poledstructure 3 e.

The quantum-entangled photon pair generator 2 c is structured so as togenerate (a) a photon pair of an e-ray (1.59 μm band) and an o-ray (1.51μm band) from a region of the period Λ₁ and (b) a photon pair of ane-ray (1.51 μm band) and an o-ray (1.59 μm band) from a region of theperiod Λ₂, each along a coaxial direction of pump light having awavelength of 775 nm at a crystallization temperature of 119.5° C.

FIG. 9 is a graph showing a temperature dependency of a parametricfluorescence spectrum in the example. At a crystallization temperatureof 108.9° C., a parametric fluorescence spectrum has 4 peaks ofdifferent polarization generated through a region of Λ₁ and a region ofΛ₂. At a crystallization temperature of 119.5° C., the peaks areoverlapped with each other such that the parametric fluorescencespectrum forms 2 peaks at 1.51 μm band and at 1.59 μm band (degenerate).Further, when the crystallization temperature is increased to 134.0° C.,the degenerate 2 peaks are split into 4 peaks again. By splitting lighthaving two wavelengths at the crystallization temperature 119.5° C. byuse of a dichroic mirror, it is possible to generate a non-degeneratepolarization-entangled photon pair that is not necessary to be selectedafterward.

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

Industrial Applicability

The present invention is applicable to a non-degeneratepolarization-entangled photon pair generation device and anon-degenerate polarization-entangled photon pair each of which generatephoton pairs in a non-degenerate

1. A non-degenerate polarization-entangled photon pair generation devicecomprising: a quantum-entangled photon pair generator including a singlecrystal in which a first periodically poled structure having a firstperiod and a second periodically poled structure having a second periodthat is different from the first period are formed; and light radiatingmeans for entering light into the single crystal such that the lightpasses through the first periodically poled structure and then throughthe second periodically poled structure.
 2. The non-degeneratepolarization-entangled photon pair generation device as set forth inclaim 1, wherein: the first period of the first periodically poledstructure is different from the second period of the second periodicallypoled structure such that a parabola indicative of a relation between anemission angle and a wavelength of a polarized photon emitted based onlight incident on the first periodically poled structure comes intocontact with a parabola indicative of a relation between an emissionangle and a wavelength of polarized photon emitted based on lightincident on the second periodically poled structure, within an allowablerange under a phase matching condition.
 3. The non-degeneratepolarization-entangled photon pair generation device as set forth inclaim 1, wherein: the single crystal is a lithium niobate crystal. 4.The non-degenerate polarization-entangled photon pair generation deviceas set forth in claim 1, further comprising: a third periodically poledstructure that produces pump light, the light radiating means enteringthe light into the third periodically poled structure such that thelight passes through the third periodically poled structure, the firstperiodically poled structure, and the second periodically poledstructure, in this order.
 5. The non-degenerate polarization-entangledphoton pair generation device as set forth in claim 4, wherein: thethird periodically poled structure is formed in the single crystal.
 6. Anon-degenerate polarization-entangled photon pair generation methodcomprising the step of: entering light into a single crystal in which afirst periodically poled structure having a first period and a secondperiodically poled structure having a second period different from thefirst period are formed, in such a manner that the light passes throughthe first periodically poled structure and then through the secondperiodically poled structure.